This invention relates to a positioning mechanism for a member that is moved linearly by the turning of a screw member driven by a stepping motor, such as is used, for example, in the positioning of a variable-throttle valve spool in a hydraulic power steering device.
In a hydraulic power steering device, a variable-throttle valve is used which comprises a spool inserted in a housing so that it can move linearly along its axial direction, a screw member that screws into the spool, a stepping motor that drives and turns a drive shaft linked to that screw member so that turning of the drive shaft can be transmitted to the screw member, means for preventing a moving member from being turned together with the screw member when the screw member is turning, a stopper capable of stopping the movement of the spool in one direction due to the turning of the screw member, and a variable-throttle portion, a degree of opening of which varies according to the movement of the spool.
In order to accurately control the degree of opening of the throttle portion in response to the turning steps of the motor, origin positioning is performed by stopping the movement of the spool by a stopper, when the motor is turning in one direction. The spool is positioned in response to turning steps of the motor in the other direction after the origin positioning, and the degree of opening of the variable-throttle portion is determined by the positioning.
Conventionally, an insertion portion, whose cross-section perpendicular to the axis is non-circular, is formed on one of the drive shaft and the screw member, and this insertion portion is fitted into a receiving portion formed on the other of the screw member or the drive shaft so that turning can be transmitted from the drive shaft to the screw member.
There is a problem in the conventional constitution that an impact sound is produced when the origin positioning is made. To be more precise, in the conventional constitution, in order to absorb errors in the precision of the concentricity between the center axis of the drive shaft and the center axis of the screw member, the insertion portion is fitted into the receiving portion with an intervening gap in the radial direction. The number of pulses sent to the motor at the time of the origin positioning is set so that some pulses are sent to the motor even after the movement of the spool has been stopped by the stopper. Thus the spool is definitely moved to a position where it makes contact with the stopper. When that is done, due to the pulses sent to the motor after the spool movement has been stopped by the stopper, the motor tries to turn by the measure of the gap in the radial direction between the insertion portion and the receiving portion. As a result, the drive shaft and the screw member impact against each other and the impact sound is produced.
An object of the present invention is to provide a positioning mechanism capable of resolving the problem noted above.
The present invention is a positioning mechanism comprising a moving member capable of linear movement, a screw member that screws into the moving member, a drive shaft fitted into the screw member so that turning of the drive shaft can be transmitted to the screw member, a stepping motor for driving the drive shaft to turn, means for preventing the moving member from turning together with the screw member when the screw member is turning, and a stopper capable of stopping the linear movement of the moving member in one direction caused by the turning of the screw member, in which origin positioning is performed by stopping, by means of the stopper, the linear movement of the moving member caused by driving the motor to turn in one direction, the moving member is positioned according to turning steps of the motor in the other direction after the origin positioning, and the drive shaft is fitted into the screw member via an elastic member interposed therebetween so that turning of the drive shaft can be transmitted to the screw member.
Drive pulses are sent to the stepping motor when the origin positioning is made. The number of the drive pulses is set so that some pulses are sent to the motor even after the movement of the moving member has been stopped by the stopper. Thus the moving member moves to a position at which it makes contact with the stopper without fail. The motor tries to turn due to the pulses sent to the motor after the movement of the moving member has been stopped by the stopper. At this time, because the elastic member is interposed between the screw member and the drive shaft driven by the motor, it is possible to prevent the occurrence of an impact noise caused by the impact between the drive shaft and the screw member.
The fitting of the drive shaft into the screw member with the intervening elastic member may be done by press-fitting or interposing a gap in the radial direction. When the press-fitting is done, the error in concentricity precision between the center axis of the drive shaft and the center axis of the screw member can be absorbed by the elastic deformation of the elastic member. The elastic member may also be separate from the drive shaft and the screw member, or it may be integrated with either the drive shaft or the screw member, or with both. A spring, rubber, or resin, etc., can be used as the elastic member.
For fitting the drive shaft into the screw member, for example, an insertion portion, of which the cross-section perpendicular to the axis is non-circular, is formed on one of the drive shaft and the screw member, and this insertion portion is fitted, so that turning transmission is possible, via the elastic member, into a receiving portion formed on the other.
It is preferable that the moving member constitutes a spool that is inserted into a housing of a variable-throttle valve, that a variable-throttle portion, degree of opening of which varies according to the linear movement of the spool along its axial direction, is provided, and that the degree of opening of the variable-throttle portion is determined by the positioning of the spool according to turning steps of the motor in the other direction after the origin positioning.
Based on this constitution, the positioning mechanism of the present invention is applied to a variable-throttle valve, and therefore an impact noise can be prevented from occurring at the time of origin positioning for determining the degree of opening of the variable-throttle portion.
It is preferable that a hydraulic control valve, which has a plurality of throttle portions having degrees of opening of which vary according to steering resistance, is comprised, that the hydraulic pressure acting on a steering assistance power generating hydraulic actuator is changed according to changes in the degrees of opening of the throttle portions, that the plurality of throttle portions in the hydraulic control valve are divided into those belonging to a first group and those belonging to a second group that are arranged mutually parallel, that steering resistance required to close the throttle portions belonging to the second group is made larger than steering resistance required to close the throttle portions belonging to the first group, that the variable-throttle valve is connected in series with the throttle portions belonging to the second group so that the ratio of hydraulic fluid flow rate controlled by the throttle portions belonging to the first group to the hydraulic fluid flow rate controlled by the throttle portions belonging to the second group can be varied, and that pulses, whose number correspond to the vehicle driving conditions, are sent from the control device to the motor.
Based on this constitution, the ratio of the hydraulic fluid flow rate controlled by the throttle portions belonging to the first group relative to the hydraulic fluid flow rate controlled by the throttle portions belonging to the second group varies in response to the vehicle driving conditions. When the proportion of the hydraulic fluid flow rate controlled by the throttle portions belonging to the second group increases, the steering torque required to close the throttle portions against the steering resistance becomes greater, and therefore driving stability can be enhanced. When the proportion of the hydraulic fluid flow rate controlled by the throttle portions belonging to the first group increases, the steering torque required to close the throttle portions against the steering resistance becomes smaller, and therefore turning performance can be enhanced. These steering characteristics can be exerted by using a variable-throttle valve in which the positioning mechanism of the present invention is employed. Accordingly, the impact noise can be prevented from occurring at the time of origin positioning for determining the degree of opening of the variable-throttle portions, and the driver can be prevented from experiencing a sense of insecurity.
It is preferable that the drive shaft is constituted of a cylindrical main body and a metal insertion portion, whose cross-section perpendicular to its axis is non-circular and has a center hole, that the main body is press-fitted in the center hole of the insertion portion, that the screw member has a receiving portion that extends in a double forked shape in its axial direction, that the insertion portion is arranged between the two opposing inner surfaces of the receiving portion, and that a rubber coating material is provided as the elastic member so as to cover at least the areas on the outer surface of the insertion portion, which come to contact with the inner surfaces of the receiving portion. Thus, torque is received by the metal insertion portion, and impact noise is prevented from occurring by the rubber coating material.
Based on the present invention, in a mechanism for positioning a moving member driven linearly by a stepping motor through a screw member, the occurrence of impact noises can be prevented when origin positioning of the moving member is performed.